@tommysg,
Your premise is faulty. The people who already like your page are also a part of the great mass of humanity on Facebook. Because they have indicated an interest in your product (let's say you're a poodle breeder and they not only like your page, they also follow the antics of an animated poodle or they belong to rescue clubs or follow blogs that offer grooming tips, even off Facebook), they are served information of interest to them.
About the only way you can better target people is with paid advertising, and then spend some time setting it up. This won't prevent all of the problem, I might add, but if you target your ad spend to only go after women or only people in the greater Des Moines area, others will be excluded from seeing the ad.
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